Ion transfer tubes are well-known in the mass spectrometry art for transporting ions from an ionization chamber, which typically operates at or near atmospheric pressure, to a region of reduced pressure. Generally described, an ion transfer tube typically consists of a narrow elongated conduit having an inlet end open to the ionization chamber, and an outlet end open to the reduced-pressure region. Ions formed in the ionization chamber (e.g., via an electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) process), together with partially desolvated droplets and background gas, enter the inlet end of the ion transfer tube, traverse its length under the influence of the pressure gradient, and exit the outlet end into a lower-pressure chamber—namely, the first vacuum stage of a mass spectrometer. The ions subsequently may then pass through apertures in one or more partitions, such apertures possibly in skimmer cones, through regions of successively lower pressures so as to be thereafter delivered to a mass analyzer for acquisition of a mass spectrum.
FIG. 1 is a simplified schematic diagram of a general conventional mass spectrometer system 10 comprising an atmospheric pressure ionization (API) source coupled to an analyzing region via an ion transfer tube. Referring to FIG. 1, an API source 12 housed in an ionization chamber 14 is connected to receive a liquid sample from an associated apparatus such as for instance a liquid chromatograph or syringe pump through a capillary 7. The API source 12 optionally is an electrospray ionization (ESI) source, a heated electrospray ionization (H-ESI) source, an atmospheric pressure chemical ionization (APCI) source, an atmospheric pressure matrix assisted laser desorption (MALDI) source, a photoionization source, or a source employing any other ionization technique that operates at pressures substantially above the operating pressure of mass analyzer 28 (e.g., from about 1 Torr to about 2000 Torr). Furthermore, the term API source is intended to include a “multi-mode” source combining a plurality of the above-mentioned source types. The API source 12 forms charged particles 9 (either ions or charged droplets that may be desolvated so as to release ions) representative of the sample, which charged particles are subsequently transported from the API source 12 to the mass analyzer 28 in high-vacuum chamber 26 through at least one intermediate-vacuum chamber 18. In particular, the droplets or ions are entrained in a background gas and transported from the API source 12 through an ion transfer tube 16 that passes through a first partition element or wall 11 into an intermediate-vacuum chamber 18 which is maintained at a lower pressure than the pressure of the ionization chamber 14 but at a higher pressure than the pressure of the high-vacuum chamber 26. The ion transfer tube 16 may be physically coupled to a heating element or block 23 that provides heat to the gas and entrained particles in the ion transfer tube so as to aid in desolvation of charged droplets so as to thereby release free ions.
Due to the differences in pressure between the ionization chamber 14 and the intermediate-vacuum chamber 18 (FIG. 1), gases and entrained ions are caused to flow through ion transfer tube 16 into the intermediate-vacuum chamber 18. A plate or second partition element or wall 15 separates the intermediate-vacuum chamber 18 from either the high-vacuum chamber 26 or possibly a second intermediate-pressure region (not shown), which is maintained at a pressure that is lower than that of chamber 18 but higher than that of high-vacuum chamber 26. Ion optical assembly or ion lens 20 provides an electric field or electric fields that guide and focus the ion stream leaving ion transfer tube 16 through an aperture 22 in the second partition element or wall 15 that may be an aperture of a skimmer 21. A second ion optical assembly or lens 24 may be provided so as to transfer or guide ions to the mass analyzer 28. The ion optical assemblies or lenses 20, 24 may comprise transfer elements, such as, for instance a multipole ion guide, so as to direct the ions through aperture 22 and into the mass analyzer 28. The mass analyzer 28 comprises one or more detectors 30 whose output can be displayed as a mass spectrum. Vacuum port 13 is used for evacuation of the intermediate-vacuum chamber 18 and vacuum port 19 is used for evacuation of the high-vacuum chamber 26.
FIG. 2 is a schematic illustration of a portion, in particular, an outlet portion 50 of a known ion transfer tube. The upper and lower parts of FIG. 2 respectively show a cross-sectional view and a perspective view of the outlet portion 50. The ion transfer tube comprises a tube member 52 (in this example, cylindrical tube) having a hollow cylindrical interior or bore 54, the flow direction through which is indicated by the dashed arrow. At the outlet end 51 of the ion transfer tube, the tube member 52 is terminated by a substantially flat end surface 56 that is substantially perpendicular to the length of the tube and to the flow direction. Further, a beveled surface or chamfer 58, which in the case of the cylindrical tube shown is a frustoconical surface, may be disposed at an angle to the end surface so as to intersect both the end surface 56 and the outer cylindrical surface of the tube member 52. The surface 58 may be used to align and seat the outlet end of the ion transfer tube against a mating structural element (not shown) in the interior of the intermediate vacuum chamber 18 or may be used so as to penetrate, upon insertion into a mass spectrometer instrument, a vacuum sealing element or valve, such as the sealing ball disclosed in U.S. Pat. No. 6,667,474, in the names of Abramson et al.
The number of ions delivered to the mass analyzer (as measured by peak intensities or total ion count) is partially governed by the flow rate through the ion transfer tube. One of the ways to increase the sensitivity of a mass spectrometer is to let in more ion laden-gas from the API source 12, provided that enough vacuum pumping is being applied to maintain a sufficient level of vacuum in the mass spectrometer for it to function. However, this approach of increasing the throughput of the ion transfer tube, either by shortening it or increasing its inner diameter, has been found experimentally to be limited by how well the solvent surrounding the ions can be evaporated during the transfer time of the tube. Thus, the ion transfer tube is generally heated to improve solvent evaporation and ion desolvation. Unfortunately, the maximum temperature that can be applied to the ion transfer tube is limited due to melting of nearby plastic parts as well as to fragmentation of fragile molecular ions such as certain peptides that may flow through the tube.
Another disadvantage arising from use of the conventional ion transfer tube is the inability of the round-cross-section bore to effectively match—and thus efficiently capture ions from—the charged particle plumes produced by linear or otherwise configured arrays of simultaneously emitting ion sources, i.e. discrete or as part of a monolithically formed microchip assembly. Arrays of multiple ion transfer tubes have been described (for example, Kelly et al., “Capillary-Based Multi Nanoelectrospray Emitters: Improvements in Ion Transmission Efficiency and Implementation with Capillary Reversed-Phase LC-ESI-MS”, Anal. Chem. 2008, 80, pp. 143-149) so as to geometrically match the charged particle plumes as well as increase the efficiency of heat transfer to the solvated ions. However, such ion transfer tube arrays are more costly to implement than is a single ion transfer tube and are difficult to align, couple to an ion source and extract and replace for cleaning. Further, an expansion of ion and gas is generally formed at the low-pressure outlet end of an ion transfer tube. If multiple ion transfer tubes are employed in an array, the ions in each of these expansions must be merged back into a single beam of ions (refocused), a process which the inventors have found to be non trivial and to lead to loss of sensitivity, as compared to the novel ion transfer tubes disclosed herein. There are therefore needs in the art for ion transfer tubes which have improved abilities to transfer heat to entrained charged particles and improved efficiency in capturing charged particles from plumes produced by emitter arrays.